1. Field of the Invention
The present invention relates to a color separating-combining optical system configured to separate light from a light source into a plurality of color spectrums, guide these color spectrums to respective image display elements, and combine the color spectrums modulated by these image display elements, and an image display optical system and projection image display apparatus using it.
2. Related Background Art
As an optical system in the projection image display apparatus such as liquid crystal projectors and the like, there is a known optical system of a three panel type in which white light emitted from a light source is separated into three color spectrums of red, green, and blue by dichroic films (dielectric films) with wavelength selectivity, the three color spectrums are transmitted or reflected, and modulated by the image display elements such as liquid crystal panels or the like for the respective colors, the color spectrums modulated are combined by dichroic films, and the combined light is enlarged and projected onto a screen or the like by a projection lens.
A conventional projection image display apparatus of this type will be described referring to FIG. 18. In FIG. 18, reference numeral 1 designates a light source, which includes, for example, a high-intensity extra-high pressure lamp, a metal halide lamp, or the like. Light exiting the light source 1 is reflected by a reflector 2 to enter a first fly's eye lens 3, which is an ensemble of lenses arranged in a grid pattern. The light is then reflected by a total reflection mirror 5 to enter a second fly's eye lens 4. Beams condensed by the second fly's eye lens 4 are then incident into a polarization changer 4 to be aligned into a certain polarization orientation. The light aligned in the polarization orientation is then condensed by a condenser lens 7 to be guided to a dichroic mirror 8, which reflects a spectrum of a blue frequency band. The dichroic mirror 8 thus separates the blue light from the incident light. This blue light passes through a concave lens 9 having the effect of shortening the path length, is then reflected by a total reflection mirror 11, and travels through a field lens 20 and an entrance-side polarizing plate 23 to enter a liquid crystal panel 26 for blue. The light passing through the blue-reflecting dichroic mirror 8 passes through a concave lens 10 and is then separated into green light and red light by a green-reflecting dichroic mirror 12, which reflects a spectrum of a green frequency band. The reflected green light passes through a field lens 19 and an entrance-side polarizing plate 22 to enter a liquid crystal panel 25 for green. The red light passing through the dichroic mirror 12 travels via field lenses 14, 18, a relay lens 16, and total reflection mirrors 15, 17, and through a red-entrance polarizing plate 21, and then reaches a liquid crystal panel 24 for red. Each light reaching the corresponding liquid crystal panel 24, 25, 26 is modulated in light intensity corresponding to an image signal in the liquid crystal panel 24, 25, 26, and thereafter passes through an exit-side polarizing plate 27, 28, or 29. Then the color spectrums are combined in a cross prism 30 having dichroic films evaporated. The light exiting the prism 30 is further enlarged and projected onto a screen (not shown) by a projection lens 31. The light spectrums reaching the blue liquid crystal panel 26, green liquid crystal panel 25, and red liquid crystal panel 24 of the three colors are defined by the spectral distribution of the light source and the cut wavelengths of the dichroic films of the dichroic mirrors 8, 12, and there is a need for provision of trimming filters for removing unwanted color light in order to enhance color purity. The trimming filter has a dichroic film (dielectric film), so that the dichroic film removes unwanted color light (unwanted wavelength band). Since the dichroic film varies its cut wavelengths, depending upon angles of incidence of light, the trimming filter is located at a position where principal rays are nearly parallel. In general, the region near the liquid crystal panel is a place where principal rays are closest to parallel rays. For this reason, the trimming filter (dichroic film) was often stuck to a substrate of the entrance-side polarizing plate or the field lens in the vicinity of the liquid crystal panel. In this case, as shown in FIG. 19, the trimming filter 32 was deposited on the flat side of the field lens 18, 19, or 20, and then the polarizing plate 21, 22, or 23 was further stuck thereonto. The polarizing plate 21, 22, 23 is cooled by air from a fan 33 disposed below the field lens 18, 19, 20.
In recent years, the liquid crystal projectors using the liquid crystal panels utilize 150 W or 200 W lamps and, in order to achieve higher intensity, higher wattage lamps tend to be used. The aforementioned trimming filter (dichroic film) absorbs heat while removing the unwanted light. The heat thus absorbed is transmitted to the polarizing plate to increase the temperature of the polarizing plate. If the temperature of the polarizing plate increases to near the durable temperature of the polarizing plate, there is a possibility of deterioration of the polarizing plate. The absorption of heat by the trimming filter (dichroic film) is thus very disadvantageous to cooling of the polarizing plate.
Incidentally, the cross prism for color combining described above is composed of a combination of four prisms with evaporated dichroic films. The cross prism is housed together with other optical components in an optical box. In a housing operation, the prism is bonded to a prism seat preliminarily made by plastic molding or made of die-cast aluminum or magnesium. Then the prism seat with the prism is attached to the optical box with screws or the like. This is because the prism seat needs to be detachably attached to the optical box.
In general, the image display elements need to be fixed at the focal position of the projection lens while being adjusted by six-axis adjustment of rotational positions with respective to the vertical, lateral, and longitudinal axes. For this reason, metal plates separate from the seat as a prism base are stuck to the prism itself, and the panels are fixed to the respective metal plates with a UV adhesive or by soldering or the like to form a prism unit. Then the prism unit is detachably attached to the optical system.
FIG. 20 shows the configuration disclosed in Japanese Patent No. 3000977. In this configuration, metal plates 59 to 62 surround the prism 53 like a scaffold, and the liquid crystal panels 54–56 and the polarizing elements and wave plates necessary for use of the panels are fixed through mounting guides 66 to the metal plates. There are also examples in which the polarizing plates are directly bonded to the prism.
In the conventional configuration as shown in FIG. 20, however, the positioning accuracy of the optical members fixed to the prism is defined by bending accuracy of the metal plates and sticking accuracy of the metal plates. Accordingly, there is a problem that it is difficult to enhance the positioning accuracy of the optical members relative to the prism. In addition, since the number of parts is large, there is another problem that assembly requires a lot of time and effort.